Two-point frequency modulation is defined by adding the contribution of a high frequency path with a digital-analogue converter and a low frequency path in a low frequency phase lock loop at the voltage controlled oscillator. During high frequency state transitions, the high frequency path is of primary importance, whereas for low frequency state transitions, the low frequency path becomes of primary importance. However, both paths contribute to modulate data frequency to cover a determined bandwidth, which extends in high modulation frequency.
EP Patent No. 0 961 412 discloses a frequency synthesizer for two point frequency modulation for transmitting data. This frequency synthesizer uses a sigma delta type modulator for data modulation by controlling a variable divider in a low frequency phase lock loop, and a DAC digital-analogue converter for high frequency modulation. This DAC converter has a variable gain, which is regulated by a digital control unit, and is controlled by a digital control signal for data frequency modulation. This digital control signal is also transmitted to the sigma delta modulator to combine the low frequency modulation with that supplied by the DAC converter.
A relative delay variation is performed in the synthesizer on the modulation signal passing through the DAC converter relative to the modulation in the low frequency loop. However, this document discloses nothing concerning adaptation of the spectrum amplitude level of the data modulated by the low frequency phase lock loop and via the DAC converter. Consequently, undesired interference may arise depending upon the state transition frequency for data transmission.
US Patent Application No. 2003/0043950 also discloses a phase lock loop frequency synthesizer using two-point frequency modulation. This frequency synthesizer also includes a low frequency phase lock loop and a sigma delta modulator, a DAC digital-analogue converter whose output acts directly on the voltage controlled oscillator VCO. Data is modulated, on the one hand at a first input of the voltage controlled oscillator VCO via the multimode divider controlled by the modulator, and at a second input of the VCO oscillator by the DAC converter. Because of the two-point modulation, this frequency synthesizer prevents the attenuation of high frequency modulated data due to the narrow bandwidth of the phase lock loop.
Even if the DAC converter gain can be adjusted, there is no provision for equalising the spectrum amplitude level of the data modulated in the low frequency loop and in the digital-analogue converter. As for the preceding synthesizer, this may lead to some distortion problems for the data modulation during state transition.
In the US Patent Application No. 2005/0046488, it is described a compensating method for a frequency synthesizer using two point modulation. The frequency synthesizer includes in particular a first low-pass phase lock loop PLL, and a high frequency path for compensating the amplitude between an analogue modulation signal and a digital modulation signal to operate two point data modulation.
The first phase lock loop of this document includes a phase frequency detector, a first charge pump, a first low-pass loop filter and a voltage controlled oscillator VCO. The voltage controlled oscillator includes in input an adder, whose a first input is connected to the first low-pass filter, followed by a frequency generation unit. A fractional-N programmable divider closes the first loop between the voltage controlled oscillator and the phase frequency detector, which receives a reference signal from a reference oscillator. This divider is controlled by a digital modulation signal provided by a modulator.
The high frequency compensating path of this document includes also to form a second phase lock loop, a second charge pump and a second low-pass loop filter connected via a switch to a second input of the adder of the voltage controlled oscillator. The second charge pump and the second low-pass filter are with a same structure as the first charge pump and the first low-pass filter. This high frequency path includes still a comparison unit, which is a comparator, whose the output is connected to an analogue modulation unit, which receives an analogue modulation signal from a digital-analogue converter.
According to the compensating method of this document, a first digital modulation signal is supplied by the modulator in order to tune the first phase lock loop to a first frequency, which corresponds to the selected carrier frequency of a transmission channel subtracted by a digital modulation amplitude. In a second phase, a second digital modulation signal is supplied by the modulator in order to operate the second lock loop while maintaining a first voltage corresponding to the first frequency on the first filter of the first disconnected loop. With this second digital modulation signal, a second frequency is synthesized that corresponds to the selected carrier frequency added with a digital modulation amplitude. As an addition is carried out by the adder of the voltage controlled oscillator, the second voltage on the second filter represents a differential signal relative to the double of the analogue modulation amplitude. So this differential signal is compared with a voltage multiplied by two provided in output of the decoupling analogue modulation unit. The compared amplitude of the signals is equalised by the comparator, which acts on the analogue modulation unit in order to adapt the amplitude of the analogue modulation output signal.
One drawback of the frequency synthesizer using two point modulation of US Patent Application No. 2005/0046488 is that there is a same gain between the first loop and the second loop. This requires providing an important place for each low-pass loop filter with a same size on an integrated circuit made for such a synthesizer. Another drawback is that the voltage controlled oscillator includes an adder in input, which allows adding the contribution of the high frequency path with the low frequency path of the synthesizer. The first and second control voltages for the oscillator are not independent of each other, but directly added for that the addition result determines the output frequency of the oscillator. This does not allow adapt a different sensitivity of the two inputs of the voltage controlled oscillator. Another drawback is that it is not the digital-analogue converter, which is directly calibrated, but an analogue modulation unit, which complicates the arrangement with the converter into the synthesizer integrated circuit, with a more important electrical consumption, because the two units are always in function. A calibration has further to be carried out for each new selected carrier frequency, which is a drawback.